The separation of nitrogen from other gases, such as oxygen, argon and hydrogen, is of considerable industrial importance. When the separation is conducted on a large scale fractional distillation is often employed. Distillation is quite expensive, however, because of the large initial capital cost of equipment and the considerable energy requirement involved. In recent times other separation methods have been investigated in efforts to reduce the overall cost of such separations.
An alternative to distillation that has been used to separate nitrogen from other gases is adsorption. For example, sodium X zeolite, described in U.S. Pat. No. 2,882,244 to Milton, has been used with some success for the adsorptive separation of nitrogen from oxygen. A disadvantage of the use of sodium X zeolite for the separation of nitrogen from oxygen is that it has low separation efficiency for nitrogen separation.
According to McKee, U.S. Pat. No. 3,140,933, an improvement in nitrogen adsorption results when some of the base sodium ions are replaced with lithium ions. This patent discloses that type X zeolite having base ions replaced by lithium ions can be effectively used to separate nitrogen from oxygen at temperatures up to 30.degree. C. Since the ion-exchange is not exhaustive and the X zeolites were synthesized using sodium as the templating agent, the partially ion-exchanged material used is mixed sodium/lithium zeolite.
U.S. Pat. No. 4,859,217 discloses that very good adsorptive separation of nitrogen from oxygen can be obtained at temperatures of 15.degree. to 70.degree. C. using a type X zeolite which has more than 88% of its ions present as lithium ions, particularly when a zeolite having an aluminum to silicon atomic ratio of 1 to 1.25 is used.
Unfortunately, lithium exchanged zeolites have very high affinities for water, and adsorbed water, even in small amounts, seriously diminishes the adsorption capacity of the zeolite. Accordingly, to secure optimum adsorption performance, it is necessary to activate the zeolite by heating it to temperatures as high as 600.degree. to 700.degree. C. to drive off as much adsorbed water as possible.
U.S. Pat. No. 5,179,979 asserts that lithium/alkaline earth metal X zeolites having lithium/alkaline earth metal molar ratios in the range of about 95:5 to 50:50 have thermal stabilities greater than the corresponding pure lithium zeolites and good adsorption capacities and selectivities.
U.S. Pat. No. 5,152,813 discloses the adsorption of nitrogen from gas mixtures using crystalline X-zeolites having a zeolitic Si/Al ratio .ltoreq.1.5 and at least binary ion exchange of the exchangeable ion content with between 5 and 95% lithium and between 5 and 95% of a second ion selected from calcium, strontium and mixtures of these, the sum of the lithium and second exchangeable ion being at least 60%.
Lithium-exchanged natural mordenite is reported to be a good adsorbent for oxygen pressure swing adsorption (PSA) in papers by H. Minato and M. Watanabe, published in Scientific Paper, University of Tokyo, (1978), 28, 218, and S. Furuyama and K. Sato in the Journal of Physical Chemistry (1982), 86, 2498-2503.
U.S.Patent 4,925,460 discloses a process for the separation of gas mixtures, where the components differ in heats of adsorption, e.g. nitrogen from air utilizing chabazite (Si/Al ratio 2.1 to 2.8) in which greater than 65% of the cations have been exchanged with lithium ion. This patent also discloses a process for the preparation of the novel adsorbent.
U.S. patent application Ser. No. 08/287324, filed Feb. 14, 1994, now U.S. Pat. No. 5,464,467, discloses the preparation of nitrogen-selective lithium- and trivalent ion-exchanged type X zeolites.
Although X-type zeolites that are highly ion exchanged with lithium ions have excellent nitrogen adsorption properties they are expensive to produce. There is a need for adsorbents which have superior thermal stability and nitrogen adsorption properties and which can be produced at reasonable costs. The present invention provides a family of adsorbents having these characteristics.